Improve efficiency of trivial toDelim calls
Improve efficiency of trivial toDelim calls

Summary:
`toDelim` with a single parameter of the same type as the target type was
previously copy-constructing its result. It will now construct it with perfect
forwarding which is more efficient if the input is a temporary.

This brings `toDelim` into line with the similar implementations of `to` and
`tryTo`

Reviewed By: yfeldblum

Differential Revision: D5301427

fbshipit-source-id: 843a2d93384de88cce42f26da6562a1a6ed0dc9c

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